Electrostatic spray painting method and apparatus



May 27, l952 c. D. TUTTLE E-r AL 2,598,466

` ELECTROSTATIC SPRAY PAINTING METHOD AND APPARATUS :inventors v 6% ff/f May 27, l952 c. D. TUTTLE ET AL 2,598,466

ELECTROSTATIC SPRAY PAINTING METHOD AND APPARATUS Filed July 15, 194s v2 SHEETS-SHEET 2 nventors Patented May 27, 1952 ELECTROSTATIC SPRAY PAINTING METHOD AND APPARATUS Charles Derwood Tuttle, Trenton, Grayland T.

Larsen, Wyandotte, and Lewis J. Lamm, Detroit, Mich., assignors tok General Motors Corporation, Detroit, Mich., a corporation of Dela- Ware Application July 13, 1948, Serial No. 38,534

4 Claims. 1

The present invention relates to the coating of articles of manufacture. lates to a method of and apparatus for spray painting articles of manufacture using an electrostatic field without the use of ionizing electrodes and utilizing an electrostatic iield between two of the articles to be coated.

Previous Workers in the electrostatic spray coating art have, for the most part, found it necessary to use ionizing dischargeelectrodes for the production of corona discharge, and/or ionization, for charging of the particles of coating material. One exception to this generally accepted method of particle charging is shown in a U'. S. patent to Pugh 1,855,869. In his proposed apparatus Pugh found it necessary to charge the spray booth to a high potential and the article to be coated to a high potential opposite in polarity. The applicantshave discovered a means of electrostatic spraying which makes it unnecessary to use any discharge electrodes or to have the spray booth maintained at a high potential. This new method of electrostatic spray painting discovered by the applicants provides a much more eicient utilization of the paint spray since the applicants spray two articles simultaneously instead of one article and a discharge electrode. Experimental work conducted by the applicants has shown thatI the attraction between paint droplets and the article to be painted and, hence the efficiency of the painting method, is a direct function of the potential impressed between a pair of articles to be coated. The applicants observed that this direct function between potential gradient and paint attraction continues to a potential of but a few volts and a potential gradient of but a few volts per inch. Thus the applicants equipment does notl depend on any ionization or corona discharge.

It is therefore an object of the present invention to provide an eiilicient method of and apparatus for spray coating articles of manufacture using an electrostatic eld.

. It is a further object of the present invention to provide a method of and apparatus for spray coatingof articles of manufacture using an electrostatic iield between two of these articles to be coated.

tion of the drawings and will bespeelncally claimedfin the claims. v y

More particularly it re- Referring to the drawings, Figure 1 is a partial sectional view of one form of apparatus used in practicing the present invention.

Figure 2 is a sectional view taken along the line 2-2 of Figure 1.

Figure 3 is a circuit for producing high voltage D. C. v

Figure 4 shows the modification of Figure 3 necessary to obtain high voltage D. C. of opposite polarity to that of Fig. 3.

Referring more particularly to Figures 1 and 2, 4 and 6 are monorail conveyors of conventional design attached to an overhead supporting structure 8 by means of supporting members I0, I2 and I4. I6 is a spray booth which may be open at both ends to permit ythe introduction and removal of the articles to be spray painted. 'I'he conveyor chains I8 and I9 have attached thereto at predetermined intervals supporting members 20, 22 and 32. These supporting members are attached to the housing members 24, 26 and 34 containing small high voltage D. C. power supplies. These power supplies will be more specifically described and discussedin connection with the Figures 3 and 4. The supporting hooks 28. 30 and 36 are mechanically connected to the housing members 24, 26 and 34 respectively but electrically insulated therefrom. These supporting hooks are electrically connected to the output terminals of the power supplies contained in these housings as will be more specifically described in connection with Figures 3 and 4. The above mentioned power supplies receive their energization through pickup coils 38, 40 and 42. The coils 42 and 40 are` energized by magnetic induction from the electromagnet 46 which is in turn energized from a cycle power source 45. The coil 38 and other inputs to power supplies carried by the conveyor 6 are energized by a similar electromagnetic source 44 which is similarly energized electrically from a 60 cycle power source. The articles to be coated, here shown as automobile doors 48, 5I! and 52, are suspended from the hooks 28, 36 and 30 respectively. These articles to be coated are thereby maintained at a high D. C. potential so that the articles suspended from the conveyor 4 are of one polarity, and the articles suspended from the conveyor 6 are of the opposite polarity. This results ina high potential electrostatic iield beingset up between the articles to be coated. The spray coating material is introduced into this electrostatic eld by means of spray guns 54, 56 and 58 of conventional design which :are supported in spaced relationship by meansof vst zaind 65.. These guns may befpointed in a dlrectiongenera-llylparallel :to the direction oi movement of the articles 48 and 52 on the conveyors 6 and 4. The guns may be so inclined in the horizontalplane as to direct the paint particles toward the articles to be coated yvvitiiout departing from the scope of this invention.

Referring more particularly to Figure 3, a conventional circuit capable of developing a suitable high D. C. potential is illustrated. Inthisfigure.. 24 is the casing or housing member shownin Figure 2. 44 is the electromagnetic member and. 38 is the coil pickup, both of` which` are shown in Figure 2. The voltage induced. in the. col, is applied to the coil 62 of the vibrator having a movable contact 18 and a stationary contact 80. During the time coil 62 is de-energized following opening of the contacts, condenser-64 charges up to the potential induced in coil 38. When. the contacts close, condenser 64 discharges through coil 66. These contacts are closed when the coil S2v is cie-energized. The interrupted current thus produced by the vibrator appears across the coil 66 and energy is induced intothe coilr 68 by the mutual inductance M betweenthe coils 56 and 68. The voltage thus produced lacross the secondary coil 68 is in theA orderofmagnitude of50 kilovolts. This alternating current is rectified by therectifier 'I0` and appears at the terminal 28 as a high positive D. C. voltage to charge the article to be coated. In order to drain the 'charge from the articles 48 and 52 after they have passed through the paint spray and the coils 38V are no longer magnetically coupled to the magnet 44, a high resistance IZ is provided.' This high resistance does-not interfere seriously with the charging of the article but allows the charge to drain on to ground after the article has left" the spray atmosphere.

Referring to Figure 4, a partial section 'oi the casing 26'and the circuit contained therein is shown. With the exception of the fact that the rectier i4 is reversed to that shown as 70 in Figure 3, these two figures are identical. The high resistance. 1.6 isA similar to and for the same purpose as the resistance 12 inFigure 3. By reversal-of the rectifier, a high negative potential is produced on the hanger 30. It may thus be seen that if a power supply such as shown in Figure 3 is-used on one conveyor and a power supply such as modified by Figure 4 is used on the opposite conveyor, the articles suspended from these two conveyors will be chargedto potentials of opposite polarity, and a strong electric eld will be set up between these articles 48 and 52.

Operation In order topaint or otherwise coat articles of manufacture utilizing the present inventionit is necessary to introduce a potential'on two of the articlesl of 'opposite polarity and spray the coating material into 'the electrostatic field thus created between these two articles. As specifically indicated in Figures 1y and 2, the articles to be painted, 48 and 52, aresuspended from the terminals 28 and 3D of the power supplies 24 and 26 respectively. Thus there is introduced apotential of one polarity on the article 43 'and a potential of the opposite polarity on the 'article 52. The power supplies 26"and 24 are mechanically attached to conveyors 4'and 6 respectively of conventional design. As` `these conveyors traveling at approximately the .same speed ap preach the spray booth, the pickup coils'i and 38 respectively enter the .electromagnetic iield produced 'between the armeni each offtheiclectr Amagnets 4S yand 44.' Thesepickup coils thereby Aeralactors and not the least being the distance between the articles 48 and 52. We have found that the attraction of the paint particles and hence the elicency of the process is a direct functi'on'of the. potential gradient between the article sj48and 52 lup to a potential gradient of several kilovolts, per inch. It is important to obtain a high potential gradient without the production of corona discharge. For this reason one of the objects of the present invention is to charge the articles to be coated at opposite polarities so that the potential of each article may be maintained as low -as possible. Thusia high potential gradient obtained with low maximum potential. As the articles of`manufacture,.48 and 524, proceed into the spray booth,'a spray mis-t of paint or other coating material: is interposed between them-by spray guns 54, and 58. These guns are maintained. at ground potential. The paint particles as they proceed .into the field produced between thetwoarticles to'be coated,are. attracted .to these articles by the electrostatic eld produced therebetween. The paint particles are thereby caused by the field-to followv a curvilinear path after they leave the guna'nd be deposited on an article tobe coated. As'several of these articles. to be coated are in the spray booth at' the same. time, thezparticles of painti which aremost strong-1y attracted-defposit on the article 'closest' to vthe spray gun and other paint particles areA deposited on other articles depending-upon theradus of curvature ofthe path'of paintparticle. Thus asA the art-icles to 'be painted progress through the spray booth, 'they are completely and eiciently coated with paint. v Y

' .As the articles pass out of the spray mist.. the pickup coils 3.8 and 42 leave the electromagnets 44 and'4 so that a potential is no longer produced on the articles 48 and 52. In order to remove the charge previously produced on these articles, a high rresistance is used to bypass-the rectiiier in the power pact. This high-resistance is lshown as Hand i6 in Figures 3 and 4 respectively. Thus as the articles'leave the spray mist, the electrostatic charge is drained from the article so that the article may proceed to the drying oven and/or be handled by operators without danger of electrical discharge with its `resulting consequences.

It is to be understood alsothatalthough. the invention has'been described with specicreference to a particular embodiment thereof-,it isf'not to beso limited, since changes andY alterations therein may be made Whichare within thelfull intended scope of this invention rias deiined 'by the appended claims.

` We claim:

1. An apparatus for coating ltWoar-ticles :of manufacture simultaneously including: conveyor meansfcapable of transporting saidfarticles in a generally parallel direction, and 'spaced transversely with respect to the path of' travelioflthe articles; means for'attaching said articlesto'said conveyor means, means for introducing a negative potential .onone of said articles 'and means forv introducing a ypositive potential en thef other of said articles, and means for 'introducing is,

for a sucient period of time to permit coating v of said articles by electrostatic attraction.

2. An apparatus for coating two articles of manufacture as claimed in claim 1 in which the means for introducing a spray of coating material is a spray gun capable of projecting coating material in a direction generally parallel to the direction of travel of said two conveyors.

3. An apparatus for coating two articles of manufacture as claimed in claim 1 in which the means for introducing the negative and positive potentials on said articles are direct current voltage supplies interposed between said conveyors and said articles of manufacture.

4. A method of electrostatically depositing particles of coating material on the extended surface of two articles of manufacture simultaneously consisting of producing a gaseous suspension of the particles of coating material, placing a negative electric potential on one of said articles and a positive potential on the other of said articles, spacing said articles apart from each other with at least a part of said extended surfaces in opposing relationship and moving said 'f articles simultaneously through said suspension of coating material in a direction generally parallel to said part of the extended surfaces opposing each other to thereby permit the said particles to be attracted to the surface of said articles by electrostatic attraction.

CHARLES DERWOOD TUTTLE.

GRAYLAND T. LARSEN.

LEWIS J. LAMM.

REFERENCES CITED The following references are of record in the vle of this patent:

UNITED STATES PATENTS Number Name Date 1,855,869 Pugh Apr. 26, 1932 2,191,827 Benner Feb. 27, 1940 2,247,963 Ransburg July 1, 1941 2,248,064 Carlton July 8, 1941 

